Permanent magnet (PM) machines are widely used in industrial applications, wind power generation systems, electric vehicles (EV), and hybrid electric vehicles (HEV). One PM machine type, called interior permanent magnet machines (IPMMs), embeds permanent magnets in the rotor. IPMMs are ideal machines for EV and HEV applications due to the high efficiency, high torque density, high power density, wide constant power range, wide speed range, and high reliability. The main drawback of IPMMs is the use of rare earth materials for the magnets, which have limited sources and are increasing in cost.
Another issue when designing an IPMM is an amount of a pulsating torque associated with an average torque production. The pulsating torque of IPMMs results from a cogging torque and a ripple torque. The cogging torque is due to the interaction between the rotor PMs and a stator reluctance at zero stator excitation. The ripple torque is due to the stator electrical loading and the rotor magnetic loading based on the interaction between the stator magnetomotive force (MMF) and the rectangular rotor PM flux distribution. The ripple torque also results from a saliency due to the interaction between the stator MMF and a non-uniform rotor permeance. The pulsating torque creates problems such as mechanical resonance, structural vibration, speed ripple and acoustic noise, drive component damage, and low machine performance under speed or position control. Significant amounts of vibration and mechanical resonant load issues can cause mechanical breakdown of the coupling shaft, resulting in the replacement of an entire shaft system at high expense.